Device for reinjecting flown-off particles into a solid fuel boiler

ABSTRACT

An apparatus for reinjecting flown-off particles into a solid fuel fire box of a boiler or furnace of the &#34;projector with back grid&#34; type, in which particulate fuel is projected from a fuel source adjacent a first zone to a grid, with the fuel burned along its trajectory from the first zone to a second zone on the grid. Means are provided for separately collecting both large and small fuel particles thrown off during combustion along the trajectory to the grid, and reinjecting the collected particles with the fine particles continuously reinjected at a rate approximately proportional to the boiler load at said second zone by means of a particle conveying air stream.

The present invention concerns a device for reinjecting flown-offparticles during combustion in the fire box of a solid fuel boiler orfurnace of the so-called "projector with back grid" type.

Such a boiler is characterised by the fact that it is fed with fuel, forexample coal having a granulometry able to reach several tens ofmillimeters, or wood, husks, bagasse, or other comparable combustiblesolids, by fuel feed means arranged adjacent a first zone of the boilerand which project continuously a determined load of fuel along atrajectory leading to a second zone of the boiler, onto a grid drivenwith a return movement from this second zone towards the first;combustion taking place during the course of the said trajectory andcontinuing not only during the end of it but also on the grid, wherethis combustion ends so that the grid drives only ashes into the firstzone, whence these ashes are evacuated.

Boilers of this type have a certain number of advantages.

With respect to boilers with a mechanical grid, in which the combustionoccurs exclusively on the grid, they have advantages connected with thefact that a part of the combustion occurs during the projection of thefuel, that is on the one hand an increase in the rate of combustion withthe resultant possibility of reduction of the surface of the grid, andon the other hand a consequent flexibility of operation, permittingrapid changes of load to be made in the best conditions.

With respect to pulverised coal boilers, boilers of this type have theadvantage of using coals of varied granulometry, and in particular coalsof a greater granulometry which does away with crushing means, which arenecessary for pulverised coal boilers and are costly in terms ofinvestment, maintenance and consumption of energy.

Nevertheless, the development of boilers with a projector and a backgrid has been limited until now, by reason of a lower output than thatof other types of boilers, and more precisely because of the veryimportant rate of combustible particles unburned.

In fact, the feed of fuel by projection permits the flying off, with theflue gases released by the combustion, of sufficiently light combustibleparticles to be thus entrained but nevertheless too heavy to burncompletely in the course of the trajectory. This inconvenience is markedwith respect to fixed grid boilers, where there is no projection, andwith respect to pulverised coal boilers, which use coal of asufficiently fine granulometry that the rate of unburned particles isminimal. In comparison with other types of boilers, an increase in theproportion of solid particles extracted from the flue gases, beforeevacuation to the atmosphere, by appropriate dust removers, with agreater proportion of carbon in the particles is noticed in the use ofboilers with a projector and back grid. In other terms, an increase oflosses and unburned solids is noted. In addition, the evacuation ofsolid particles extracted from the flue gases by the dust removers canpresent difficulties on account of their number.

For remedying these inconveniences of projector and back grid boilers,it has been proposed to reinject into the boiler a part of the solidparticles flown off with the flue gases, after they have been caught atthe outlet from the boiler by means of dust removers or separators usedfor purifying these flue gases before their being ejected into theatmosphere.

In practice, such dust removers or separators are normally provided inseries, for extracting from the flue gasses first the larger particlesand then the finer particles. Up to now the larger particles separatedfirst have been reinjected, but the finer particles have not also beenrejected, since the fine particles are particularly difficult to burnbefore they fly off again, carried by the flue gasses. In other terms,up to now one has been limited to a reinjection of the larger particlesinsofar as there exists a significant risk that the reinjection of thefiner particles will lead immediately to their flying off again, with orwithout combustion, together with the finer particles of the loadintroduced by the projector, with the result of a rapid coking up of theinstallation.

The object of the present invention is to eliminate such a risk, bypermitting a total reinjection of the solid particles drawn off by thedifferent successive dust removers or separators, including the finestparticles separated immediately before ejection of the flue gases to theatmosphere.

For this, the process according to the invention, consists in a knownmanner of drawing off in a projector and back grid boiler the flue gasesreleased by the combustion, entraining the solid particles, leading themthen successively into means for separation of larger particles and intomeans for separation of finer particles, and evacuating the flue gasesafter this separation whilst the separated particles are reinjected intothe boiler, characterised in that all the separated particles arereinjected into the boiler,

in a manner possibly known insofar as it concerns the larger particlesand,

insofar as it concerns the finer particles furnished by correspondingseparation means in an irregular flow, by means of operations consistingof:

(a) transforming this irregular flow into a continuous flow ofparticles, at least approximately proportional to the load of theboiler,

(b) introducing continuously this continuous flow of particles into acontinuous flow of conveying air,

(c) leading these particles continuously into approximately to thesecond zone of the boiler, and;

(d) injecting them into this second zone, into a part of the saidtrajectory close to the grid.

In thus reinjecting the fine particles into the project precisely therewhere the finer particles of it are burning, the burning of thereinjected particles is facilitated and, in choosing that part of thetrajectory into which this reinjection is made, the trajectory partclosest to the grid, entrainment of the reinjected paticles isfacilitated. Deposit occurs in the intersection zone of the grid withthe projection trajectory of the fuel and this zone constitutesprecisely the hottest zone of the grid, which favours incineration ofthe reinjected particles being thus burned, that is to say the formationof ashes of which the flying off is no longer to be feared, and whichcan be evacuated with the other ashes when they reach the first zone ofthe boiler, as a result of movement of the grid.

It should be noted that this flow, of fine particles, from thecorresponding separation means can be very irregular, for example onaccidental or voluntary discharge of the particles after significantvariation of the load of the boiler. Transformation of this irregularflow into continuous flow approximately proportional to the load of theboiler, allows the combustion to be undisturbed by the reinjection inthis boiler, that is to say of reinjecting at all values of load withoutirregularity of heat, whatever the perturbations which can affect theinstantaneous flow rate of the means for separating the fine particles.

Naturally, the flow rate of conveying air must be such that this airdoes not greatly disturb the combustion inside the boiler, andparticularly does not disturb the combustion of reinjected particlesthus conveyed; taking account of the high carbon content of theseparticles and of their very low content of volatile material, it isconvenient that the concentration of reinjected fine particles withrespect to the air which conveys them should be sufficiently high, andone obtains good results with a ratio of mass flow rate of fineparticles to mass flow rate of conveying air of the latter between 1 andapproximately 10, these figures being given by way of non limitativeexample.

Further, the volume flow rate of conveying air is advantageouslysubstantially constant, although adjustable, only the flow rate of thefine particles in this air varying, in order to assure a regular speedof injection.

Thus, the process according to the invention permits reinjection of allof the solid particles drawn off from the flue gases before evacuationof the latter to the atmosphere, and burning of the combustible part ofthese particles in the best conditions, this permits achievement ofappreciable economies of combustion without causing elsewhere acomplication of the installation; there results an optimal utilisationof the fuel, in all respects comparable to that which one achieves froma pulverised coal boiler, without it being necessary to provide acrusher, a particular inconvenience of such boilers.

It should be noted in addition that the total reinjection permits onlyextraction of the waste, in practice the ashes in a single zone and in aconvenient form, easy to be retreated.

For putting this process into use, the present invention proposes inaddition a device comprising:

means for drawing off the flue gases in the boiler,

means for evacuation of the flue gases,

first separation means for the separation of particles,

second separation means for the separation of particles,

means leading the flue gases from the drawing off means to the firstseparation means, from the first separation means to the secondseparation means, from the second separation means to the evacuationmeans of the flue gases,

means for drawing off particles in the first separation means and ofreinjecting such particles into the boiler,

means for drawing off particles in the second separation means,

this device being characterised in that the means for drawing offparticles in the second separation means comprises:

(a) a buffer storage,

(b) means for discharging particles from the second separation meansinto the buffer storage, preventing direct communication between theselatter,

(c) means for continuous drawing off of particles in the buffer storage,with an adjustable flow rate,

(d) means for controlling in accordance with the load of the boiler theflow rate of the means for continuous drawing off of particles in thebuffer storage,

and in that there are provided:

a source of air under pressure,

injection means arranged in the proximity of the second zone of theboiler and opening towards a part of the said trajectory close to thegrid in this second zone,

a pneumatically conveying duct connecting the source of air underpressure to the injection means, the means for continuous drawing off ofparticles in the buffer storage opening into the said duct.

In an advantageous embodiment of the device, the pneumatic conveying ismade by the connection between, on the one hand, the means ofdischarging of particles from the second separation means into thebuffer storage, and, on the other hand, this latter, which permitsdissociation of these means for discharging and in particular thejuxtaposition of these latter, that is to say not positioning themimmediately below; for this, the device has a second source of air underpressure, a second pneumatic conveying duct connecting this secondsource to the buffer storage, the means for discharging of the particlesfrom the second separation means into the buffer storage opening intothis second duct preventing a direct communication between this latterand the second separation means.

Advantageously, it can then be provided that the source of air underpressure initially mentioned, intended to supply the pneumatic conveyingduct leading the means for continuous drawing off of particles into thebuffer storage by means of injection into the boiler, is constituted byan upper part of the buffer storage; in other terms, the same conveyingair is used for successively leading to the buffer storage the particlesfrom the means for discharging, and then to the boiler particles fromthe buffer storage.

Further, when the second separation means comprises a plurality ofseparators connected in series and/or in parallel, between the firstseparation means and the means for evacuation of the flue gases, via theflue, it is then possible to provide a discharging of all of theseseparators into a single buffer storage without the need to incorporatein the latter the plan dimensions corresponding to those of the assemblyof the second separation means thus constituted; the device according tothe present invention is then characterised in that there is providedmeans for discharging of particles from each of the separators into thesingle buffer storage, these means for discharging opening into the saidsecond duct, which is common, preventing direct communication betweenthis duct and the separators.

This solution is advantageous not only in terms of overcrowding, butalso in terms of simplification of the means used for the adjustment offunctioning, from the simple fact of the unique character of the bufferstorage.

Whilst, when the second separation means have a plurality of separatorsconnected in series and/or in parallel, between the first separationmeans and the means for evacuation of the flue gases, via the flue, themeans for drawing off of the particles in the second separation meanscan equally be provided so as to comprise:

(a) a plurality of buffer storages of which each is associated with atleast one separator,

(b) means for discharging particles from this separator into theassociated buffer storage, preventing a direct communication betweenthese latter,

(c) means for continuous drawing off of particles in each bufferstorage, with an adjustable flow rate,

(d) means for controlling to the load of the boiler the flow rate ofeach of the means for continuous drawing off of particles in one of thebuffer storages, and that the means for continuous drawing off ofparticles in different buffer storages open into the mentioned pneumaticconveying duct, which is common.

Thus drawings off of particles into each buffer storage are able to bemade which are both regular and apt to the mean production of theassociated dust remover.

Advantageously, the means for controlling the flow of the means forcontinuous drawing off particles into the or each buffer storage to theload of the boiler comprise means for controlling this flow to maintaina mean level of particles in this buffer storage, and which permit theprogressive absorption, without perturbation of the reinjection and thecombustion of particles in the boiler, possible sharp variations of theload of particles received by the buffer storage as a result of therepercussion, with delay, of sharp variations of the load of the boiler,or again a discharge of the second separation means and more precisely,when these latter have several dust removers, of a discharge of one ofthese dust removers or several of them.

Other characteristics and advantages of the process according to theinvention and of the device proposed for putting it into effect willappear from the following description of non-limitative examples, aswell as from the accompanying drawings which form an integral part ofthis description.

FIG. 1 shows the layout of a boiler with a projector and a back grid,equipped with a reinjection device putting into effect the processaccording to the invention.

FIG. 2 shows a layout of a boiler with a projector and a back grid,equipped with a variant of the reinjection device according to theinvention.

FIGS. 3 and 4 illustrate two variants of branching of second separationmeans, in the body of this variant of the device.

Referring first to FIG. 1, where is designated by 1 a coal boiler,having internally a fire box 2 delimited below by an approximatelyhorizontal grid 3 constituted by an endless conveyor 4 traversing fromside to side of the boiler 1, approximately horizontally, and runningaround respectively from side to side over rolls 5,6 which particularlydefine in the conveyor 4 an upper side 7, approximately horizontal, ofwhich an intermediary zone between the rolls 5 and 6 constitute the grid3. Motor means (not shown) drive the conveyor 4 so that its upper side 7forming the grid 3 provide approximately horizontal translation in thedirection of arrow 8.

A fuel source such as coal feed means 10 is arranged in a first zone atthe downstream end of grid 3 in firebox 2. Feed means 10 open into firebox 2 and are formed with a storage hopper 11 outside the fire box ofboiler 1 opening downwards above an endless conveyor 12 also outside theboiler 1, which has an upper side 13 approximately horizontal receivingthe coal 14 from the storage hopper 11. Motor means 16 drive the endlessconveyor 12 in a movement such that the upper side 13 is displaced inthe direction indicated by arrow 15 carrying the coal 14 to projectordevice 17 disposed above the first zone 9 of the grid 3 and havingblades 18 which a motor not shown drives in rotation about a horizontalaxis spaced from fixed peripheral grid 19. Thus the coal driven by theupper side 13 of the conveyor 12 as far as the edge of the boiler 1falls on to the device 17 and the latter projects this coal into thefire box 2, in a trajectory 20 hitting the grid 3 in a second zone 21which constitutes its upstream zone with reference to the direction 8.In other words, the coal introduced by the projector device 17 crossesthe fire box 2 from side to side for deposit on grid 3 in the secondzone of the fire box 21 opposite the first zone 9 where the coal orother fuel is introduced into the fire box. The volumetric flow rate ofcoal 14 from the hopper 11 is adjusted by adjustment of the speed ofdisplacement of the upper side 13 of the conveyor 12 in the direction15, that is to say by adjustment of the output speed of the motor 16,the blades 18 being driven in rotation about their horizontal axis at aspeed chosen as a function of the trajectory 20 to be achieved, so thatit is defined as above.

Combustion of the thus introduced coal into the fire box 2 commencesduring the span of the trajectory 20 and continues on the grid 3, helpedby the injection of primary air into the fire box 2 via a casing 22opening into the fire box 2 under the upper side 7 of the conveyor 4,that is to say under the grid 3, and by injection of secondary air viapipes such as 23,24 opening into the fire box 2, in faces 108,109 of theboiler corresponding respectively to the upstream 21 and downstream 9zones of the grid 3, at an intermediary level between that of the grid 3and that of the projector device 17 as well as, preferably, at a higherlevel than that of the projector device 17, and close to this level.

The speed of displacement of the grid 3 in the direction 8 isestablished so that the coal on this grid in the upstream zone 21 of itis reduced to a state of ashes on its arrival at the downstream zone 9,this ash being evacuated by gravity on turning, by the conveyor 4, ofdeviation means 6 placed downstream with respect to the direction 8, asshown at 25.

The combustion of coal during the span of the trajectory 20 and on thegrid 3 causes a release 26 of flue gases which the walls 27 of theboiler, delimiting the fire box 2 laterally and above, guide togethertowards an approximately horizontal duct 28, causing them to traverse anevaporator 29 comprising an array of vertical tubes connecting a lowerdrum 30 to an upper drum 31 for vaporising a liquid totally filling thelower drum 30 and the array of tubes, and partially the upper drum 31;the latter is connected above the liquid level to an outlet collector 32of the vapour from the boiler, via the intermediary of a superheater 34placed on the constrained passage of the flue gases, and below the levelof liquid to a water inlet collector 33 in the boiler, via theintermediary of an economiser heat exchanger 35 also placed on theconstrained passage of the flue gases.

The output speed of the motor 16 is controlled in accordance with theflow of vapour to satisfy the needs of the user, or the load of theboiler.

Boilers of this type are well known to the man in the art, who knows themanner of practical realisation of the different elements which havebeen described.

The duct 28 leads the flue gases drawn off in the boiler 1 successivelyto first separation means 36 intended to separate the larger particles,then to second separation means 43 intended to separate the finerparticles before leading the flue gases thus freed of dust towards meansfor evacuation to the atmosphere, shown as 44.

The first separation means 36 can be constituted by any known device,adapted to carry out larger dust removal; they can be constituted forexample by a mechanical dust remover, for example a centrifuge, or bythe first field of an electrostatic separator.

As is already known in itself, means are provided for drawing off inthese first separation means 36 the particles separated by these latterand reinjecting them into the boiler 1; in the preferred example ofoperation illustrated, where the only detail indicated of these firstseparation means 36 is a lower hopper 37, these means of drawing off andof reinjection comprise a vertical duct 38, provided with two juxtaposedvalves 39,40 and into which the hopper 37 opens downwards, this duct 38itself opening downwards into an intermediate zone of a horizontal duct84 of a pneumatic conveyor connecting a source of air under pressure 42to the fire box 2 of the boiler 1, into which this duct 84 opensapproximately horizontally, as is indicated at 41, above the upstreamzone 21 of the grid 3, at a level corresponding approximately to that ofthe projector 17 or at a lower level, so that the particles thusreinjected at 41 inside the boiler 1 are collected by the coal projectedon the trajectory 20 by the projector 17, and then follow thistrajectory with the thus projected coal.

The parameters of this reinjection of the larger particles separatedfrom the flue gases in the means 36 can be easily determined by the manof the art; one can moreover choose other means, already known, ofreintroduction of such particles into the fire box, as for example areintroduction by the projector 17, taking into account the granulometryof the particles thus reinjected at 41, the combustion of theseparticles without their flying off, together with the coal introduced onthe trajectory 20 by the projector 17, does not pose particular problemsindicated above, connected with the reinjection of particles of a finergranulometry, and for it one resorts to the present invention.

It should be noted that all the larger particles separated from the fluegases by the first separation means 36 are thus reinjected at 41 intothe fire box 2; the means permitting also reinjection of the totality ofthe finer particles separated later, in the second separation means 43to which the duct 28 leads the flue gases after they have had removedfrom them the larger particles in the first separation means 36 andbefore being evacuated to the atmosphere by the means 44, will now bedescribed.

By way of non-limitative example, the case is illustrated where thesecond separation means 43 are constituted by three separators 45,46,47,which the flue gases traverse successively in this order, in series,there losing particles respectively more and more fine collected in alower hopper respectively 48,49,50 of these separators 45,46,47; theseseparators can be either fields of the same electrostatic dust remover,or dust removers of a different type.

Each of these hoppers 48,49,50 opens downwards onto a respective valve51,52,53 able to close in a gas-tight manner or to open for permittingthe descent, by gravity, of the solid particles collected.

Under each of the valves 51,52,53 is disposed a respective intermediaryhopper 54,55,56, fluid-tight, having an interior volume such that oneach opening of the associated valve 51,52,53, it can receive the entirecharge of solid particles from the lower hopper 48,49,50 of theassociated separator 45,46,47.

For this, in use, an opening then closing of each valve 51,52,53,normally closed, for emptying the lower hopper 48,49,50 of thecorresponding separator is carried out either when the latter contains apre-determined volume of particles, as a function of which is chosen thevolume of the intermediate associated hopper 54,55,56, or cyclicallywith a periodicity chosen such that the volume of particles in thislower separator hopper never exceeds this pre-determined volume.

Each of the intermediary hoppers 54,55,56 opens downwards onto a valve57,58,59 in all respects similar to the valves 51,52,53.

Inside each of the intermediary hoppers 54,55,56, at the bottom of thelower part of it, opens a respective duct 100,101,102 branched off ontoa duct 97 which will be described below, and which carries air underpressure from a volumetric compressor 98; each of these ducts100,101,102 permits injection into the associated intermediary hopper54,55,56, fluidisation air for the particles in it, the flow of this airbeing able to be regulated individually by an appropriate valve 103 fromthe duct 100, 104 from the duct 101, 105 from the duct 102.

The particles are thus held, in each of the intermediary hoppers54,55,56 in a state of fluidity such that they can be easily poured outdownwards when the valve 57,58,59 is open.

Downwardly, each valve 57,58,59 opens onto a respective vertical ductfor evacuation by gravity 94,95,96 and the different ducts 94,95,96themselves open downwardly into the approximately horizontal duct 97mentioned above, in positions spaced along it downstream of the zonewhence lead off the ducts 100,101,102 for fluidisation air withreference to the direction 99 of air circulation in this duct 97,imposed by the volumetric compressor 98; a diaphragm 106 is interposedin the duct 97 between the mouth of the different ducts 94,95,96 and theopening of the ducts 100,101,102 for causing a passage of air into thelatter.

Thus, the air carried by the duct 97 in accordance with a flowadjustable by adjustment of the volumetric compressor 98 cansuccessively carry along the drawn off particles in the intermediaryhopper 56 when the valve 59 is open, and which fall via the duct 96, thedrawn off particles in the intermediary hopper 55 when the valve 58 isopen, and which fall via the duct 95, and the drawn off particles in theintermediary hopper 54 when the valve 57 is open, and which fall via theduct 94; it should be noted that this order, chosen by way of example,is not a characteristic of the invention and is not limitative of it.

Upstream of the assembly of the ducts 94,95,96 with reference to thedirection 99, the air circulating in the duct 97 carries in thisdirection 99 the assembly of particles thus collected as far as theupper part 107 of a single buffer storage 60, fluid-tight, delimiting aninternal volume greater than the sum of the respective volumes of theintermediary hoppers 54,55,56 so that it can always contain a volume ofparticles greatly superior to the volume which can arrive from theintermediary hoppers 54,55,56 when the valves 51,52,53 connecting thesewith the respective associated separators 45,46,47 are opened; inaddition the volume and shape of the buffer storage 60 is such that,when it receives, via the pneumatic conveyor duct 97, from theintermediary hoppers 54,55,56 a charge of solid particles by opening ofthe valves 57,58,59, there follows in the buffer storage a smallvariation of level of the load of solid particles in it.

The practical arrangements which can be adopted for this can be variedto a great extent, and will be chosen by the man in the art withoutdeparting from the scope of the present invention.

For example, the buffer storage 60 has a lower part in the form of ahopper progressively narrowing downwards, and an upper part 107 ofconstant cross-section in a horizontal plane, the lower part beingintended to be permanently full of particles throughout its height, aswell as the upper part 107 in part of its height.

With the buffer storage 60 is also associated a mean upper level 63 ofits load of particles; a level detector 91, associated with the bufferstorage 60, permits detection and either quantification, or comparisonwith a predetermined threshold or several predetermined thresholds, thepossible differences between the actual level of particles in the bufferstorage and the predetermined mean level 63, corresponding to thisbuffer storage; such detectors are known to the man in the art.

Each intermediary hopper 54,55,56 constitutes an air lock permitting thepassage of particles from the lower hopper 48,49,50 of the respectiveassociated separator 45,46,47 to the buffer storage 60, via the duct 97,whilst preventing direct communication, with the possibility of passageof gas, between the internal volume of this buffer storage and theseparators 45,46,47; for this, in use, each of the valves 51,52,53 isonly opened on the condition that the valve 57,58,59 associated with thesame intermediary hopper 54,55,56 is closed, and each of the valves57,58,59 is only opened on the condition that the valve 51,52,53associated with the same intermediary hopper 54,55,56 is closed; inpractice, an opening then closing of each valve 57,58,59, normallyclosed, for emptying the associated intermediary hopper 54,55,56 occursafter each opening and closing of the corresponding valve 51,52,53.

Other means can naturally be chosen for permitting the passage of solidparticles collected by one of the separators 45,46,47 to the bufferstorage 60, but the choice of such an air lock has permitted theobtaining of entire satisfaction in the operating conditions of thedevice, that is to say taking into account that the solid particlesconsidered are present in the form of dust.

Inside the buffer storage 60, at the bottom of the lower part of it,opens a duct 85 which permits injection into the buffer storage 60 offluidisation air for the particles in it, the flow of this air beingable to be adjusted by an appropriate valve 88 in the duct 85; this aircoming for example from the source 42, the duct 85 being then branchedoff from the duct 84, between this source 42 and the mouth of the duct38, in a manner not shown but analogous to that which has been describedwith reference to the ducts 100,101,102 and 97.

The particles are thus maintained, in the buffer storage 60, in a fluidstate such that they can be easily drawn off by drawing off means at acontinuous, adjustable flow rate, onto which this buffer storage 60opens downwards; these drawing off means have been designated by 69 andadvantageously are constituted by a rotary air lock or cellulardistributor, having as is known a plurality of blades driven in rotationabout an axis, by a motor 72, inside an envelope with which the bladesdelimit cells which the rotation of the blades puts into communicationalternatively with the buffer storage 60, upwards, and, downwards, withthe vertical duct 75 for evacuation by gravity; the flow rate of such acellular distributor, in terms of volume flow rate or mass flow rate, iscontrolled by the speed of rotation of the blades, that is to say bytheir speed of driving by the associated motor 72.

Downwards, the duct 75 opens into an approximately horizontal section ofa duct 66 which takes air under pressure, supplied from the volumetriccompressor 98 via the duct 97, into the upper part 107 of the bufferstorage 60 and carries this air in a circulation direction 78; athrottle 68 is interposed in the duct 66, between its opening into theupper part 107 of the buffer storage 60, and the opening of the duct 75into this duct 66, for establishing at the opening of the duct 75 apressure lower than that present in the upper part 107 of the bufferstorage 60.

From this, the air carried by the duct 66, according to a flow rateregulated by the volumetric compressor 98, carries along the particlesremoved in the buffer storage 60 according to a flow rate predeterminedby the cellular distributor 69, and which fall via the duct 75.

Downstream of the connection of the duct 75, with reference to thedirection 78, the air circulating in the duct 66 carries in thisdirection 78 the particles thus collected as far as the injection means79 of a type known in themselves, used for the injection of dustymaterials into boilers, which injection means 79 open into the fire box2 approximately horizontally, above the upstream zone 21 of the grid 3,at a level which is intermediary between the levels of the pipes 23,24for injection of secondary air and correspond at least approximately tothe level of injection 41 of the larger particles separated by the firstseparation means 36; the injection means 79 are directed towards thetrajectory 20, and more precisely towards a part of it close to the gridin the upstream zone 21 of this, for favouring the carrying along of thefine particles thus injected at 79 by the coal projected by theprojector device 17 on the trajectory 20, and the following of thistrajectory as far as the grid 3 by these fine particles.

Conforming with the present invention, the flow of the conveyor air forthe particles in the duct 66 and the flow of particles in this air, viathe drawing off means in the buffer storage 60, here constituted by thecellular distributor 69, are continuous, and the flow of particlesupstream of the opening of the duct 75 into the duct 66, expressed interms of mass flow rate or volume flow rate, is at least approximatelyproportional to the load of the boiler, for example to the flow rate ofthe feed means 10 expressed in the same units, which is representativeof this load.

For this, the flow rate of the drawing off means in the buffer storage60, that is to say the cellular distributor 69, is controlled inaccordance with the load of the boiler in a manner to be at leastapproximately proportional to it.

Taking account of this, in the steady state, substantially constant loadof the boiler and for a coal of predetermined characteristics, the flowrate of solid particles collected in the dust removers 45,46,47 then ledto the buffer storage 60 is substantially proportional to the feed rateof the boiler with coal 14 from the hopper 11, itself representative ofthe load of the boiler, it has been provided for this, in the embodimentillustrated, a control of the motor 72 to information furnished by thelevel detector 91, in a manner to limit the variations of the level ofparticles in the buffer storage 60 in comparison with the predeterminedmean level 63; it should be noted that thus in addition is assured fromthis that the drawing off means 69 receive particles, in the bufferstorage 60, an approximately constant force permitting them to work inconditions themselves approximately constant, independently of therespective emptyings of the intermediary hoppers 54,55,56.

The means for permitting control as to the speed of the motor 72 to theinformation furnished by the level detector 91 has been shown by a chaindotted line 81; they can be chosen by the man of the art within a largerange of possibilities without departing from the scope of the presentinvention, as a function particularly of the type of level detector 91utilised in accordance with the case a step by step correction or thepossibility of a continuous correction.

For example, in accordance with the preferred embodiment, the leveldetector 91 permits detection of the passing of the actual level ofparticles in the storage 60 at two different levels, by reason of abottom level 63B and a top level 63H of which the mean defines the meanlevel 63, and emitting at an adjustable interval impulses representativeof the one of these two levels which is actually reached by theseparticles; the control of the rate of the cellular distributor 69, thatis to say the speed of the motor 72 of it, to the information thusfurnished by the detector 91 can be carried out in the following mannerin the case:

on the starting of the installation, the buffer storage 60 beingpresumed to be initially empty, and until the top level 63H is reachedfollowing successive emptyings of the intermediary hoppers 54,55,56 intothe storage buffer 60, the motor 73 is driven at a predetermined minimumspeed of rotation, which corresponds to a reinjection of particles at 79at a minimal flow rate;

the level 63H having been attained, which is confirmed by the sending,by the detector 91, of a predetermined number of corresponding pulses,the control means 81 causes an increase in the predetermined value ofthe speed of rotation of the motor 72; if, then, a same predeterminednumber of pulses sent by the detector 91 testify to the fact that thelevel 63H is still reached or exceeded, the control means 81 cause afurther increase in speed of the motor 72, to the same predeterminedvalue, and this process of increase of the speed of the motor 72 iscontinued until the actual level of particles in the buffer storage 60redescends below the top level 63H, to which the pulses from thedetector 91 testifies;

when the top level 63H is thus dropped below, the actual level ofparticules nevertheless remains above the bottom level 63B, the controlmeans 81 hold constant the rotation speed of the motor 72;

if the actual level of the particles in the buffer storage 60 risesagain to reach once more the level 63H, the recited process startsagain;

if the level in the buffer storage 60 redescends below the bottom level63B, the emission by the detector 91 of the said predetermined number ofcorresponding impulses causes, by the control means 81, a reduction ofthe rotation speed of the motor 72, in accordance with the predeterminedmentioned value; this process can be repeated either until the bottomlevel 63B is once more reached, and then interrupted, or until thementioned minimum speed is reached, if the actual level of particles inthe buffer storage 60 does not reach again the bottom level 63B;

particularly, on stopping of the installation, the dropping below thebottom level 63B leads to the rotation speed of the motor 72 being thementioned minimum speed, which returns the installation to its initialstate.

In addition, a detection can advantageously be provided for the possiblepassage of the level of particles, in the buffer storage 60, above asafety level 63S higher than the level 63H, by means of the detector 91or another level detector, with a control such that the passing of thislevel 63S stops the extraction of particles in the intermediary hoppers54,55,56 and their pneumatic conveying, via the duct 97, as far as thebuffer storage 60, this extraction and this conveying automaticallyrestarting when the safety level 63S is once more descended from.

Advantageously, for permitting an absorption of variations in thequantity of particles received by the dust removers 45,46,47consecutively to the repercussion, with delay, of a significantvariation in the load of the boiler or again to a discharge of thesedust removers, with perturbation of the conveying via the duct 66 andwithout the reinjection into the fire box at 79 causing excessivevariations in the release of heat, an arrangement can be provided foradjustment of the output speed of the motor 72 as a function ofinformations provided by the level detector 91, by means of a tendencysignal representiative at each instant of the load of the boiler andwhich is exploited in the direction of a proportionality of flow rate tothe means of drawing off into the buffer storage 60, that is to say ofthe cellular distributor 69, to this load; the means used for this,which can be chosen by the man of the art from a large range ofpossibilities and as a result are shown only by a chain dotted line 80,tend for example to connect in a ratio of predetermined proportionality,as a function of the quantities of solid particles waiting in the dustremovers 45,46,47 for the determined loads of the boiler taking accountparticularly of the characteristics boiler taking account particularlyof the characteristics of the coal used, the speed of rotation of themotor 72 to that of the motor 16, which is representative of the load ofthe boiler.

A regular reinjection of particles is thus assured.

It should be noted that the manner of control of the flow rate of themeans for drawing off into the buffer storage 60 to the load of theboiler, in the direction of at least approximate proportionality, whichhave been described, give priority to the detection of the level ofparticles in the buffer storage 60 and intervenes only in terms oftendency of the load of the boiler at the instant considered, can bereplaced by a manner of control in the direction of such proportionalitywhich will be described below with reference to FIGS. 2 to 4,intervening firstly to the load of the boiler and to the correction ofthe detection of the level in the buffer storage or in each bufferstorage;

on the other hand, the manner which has been described can be adaptedfor the assembly or for each of the buffer storages which will bedescribed with reference to FIGS. 2 to 4.

The flow of particles into the duct 66 being thus determined, the flowof conveying air in this duct, preferably constant in terms of volumeflow rate, is adjusted by action on the volumetric compressor 98 suchthat the mass flow rate of particles introduced into the duct 66 is in aratio to mass flow rate of air in this duct of between 1 andapproximately 10; these figures, given by way of non limitative example,correspond to a high concentration of suspension of particles in airinjected at 79 into the boiler, such a high concentration beingfavourable to the combustion of these particles on their arrival in theboiler and their incineration into the form of ashes once they areburned and arrive on the grid 3.

Illustrated by chain dotted lines on FIG. 1, are two variants of thedevice which have been described.

These two variants have as a common characteristic that instead of beingsupplied with air under pressure by the volumetric compressor 98, viathe duct 97 and the upper part 107 of the buffer storage 60, the duct 66ensuring the pneumatic conveying, towards the injection means 79, ofparticles drawn off in the latter by means 69 is fed via a blower (anon-illustrated variant) or by the same blower 42 as the duct 84 as isillustrated at 66a; then, the air introduced into the upper part 107 ofthe buffer storage 60 by the volumetric compressor 98 can be eitherevacuated as free air, as is shown at 66b after filtering by appropriatemeans, or more advantageously be reinjected into the second separationmeans 43, as is shown at 66c.

Referring now to FIG. 2, where will be found under the same references,identical both as to their nature and cooperation, elements 1 to 59 and84 of FIG. 1, possibly shown in a more schematic manner.

This varied embodiment of the device differs from that of FIG. 1 in thateach valve 57,58,59 opens downwards on to the respective buffer storage360,361,362 in a fluid tight manner, delimiting an interior volumegreater than that of the associated intermediary hopper 54,55,56 so thatit can contain permanently a volume of particles greatly superior to thevolume which can arrive in the associated intermediary hopper 54,55,56when the connecting valve 51,52,53 of it with the associated separator45,46,47 is open; in addition, the volume and form of each bufferstorage 360,361,362 are such that when it receives from the associatedintermediary hopper 54,55,56 a load of solid particles by opening of theconnecting valve 57,58,59, there follows in the buffer storage a smallvariation of level of load of particles in it.

The practical arrangements able to be adopted for this can be varied ina large measure, and will be chosen by the man of the art withoutdeparting from the scope of the present invention.

For example, each of the buffer storages 360,361,362 has a lower part inthe form of a hopper, progressively narrowing downwards, and an upperpart of constant cross-section in a horizontal plan, the lower partbeing intended to be filled permanently with particles throughout itsfull height as is the upper part through part of its height.

To each buffer storage 360,361,362 is thus associated with an upper meanlevel 363,364,365 of its load of particles; a level detector 391,392,393respectively associated to each buffer storage 360,361,362 permittingdetection and either quantification, or comparison with a predeterminedthreshold or several predetermined thresholds, the possible differencesbetween the actual level of particles in the buffer storage underconsideration and the predetermined mean level 363, 364,365corresponding to this buffer storage; such detectors are known to theman in the art.

Each intermediary hopper 54,55,56 constitutes an air lock permitting thepassage of particles from the lower hopper 48,49,50 of the respectivelyassociated separator 45,46,47 to the corresponding buffer storage360,361,362 without at any instant, the internal volume of this latterbeing put into direct communication, with the possibility of passage ofgas, with the separator 45,46,47; for this, in use, each of the valves51,52,53 is only opened on the condition that the valve 57,58,59associated with the same intermediary hopper 54,55,56 is closed and eachof these valves 57,58,59 is only opened on the condition that the valve51,52,53 associated to the same intermediary hopper 54,55,56 is closed;in practice, an opening and then closing of each valve 57,58,59,normally closed, for emptying the associated intermediary hopper54,55,56 intervenes after each opening and closing of the correspondingvalve 51,52,53.

Other means can naturally be chosen for permitting the passage of solidparticles collected by one of the separators 45,46,47 to the respectiveassociated buffer storage 360,361,362 but the choice of such air lockspermits obtaining entire satisfaction in the operating conditions of thedevice, that is to say taking account of the solid particles inconsideration being in a dusty state.

At the interior of each of the buffer storages 360,361,362 at the bottomof the lower part of it, opens a respective duct 385,386,387 branchedonto the duct 366 which will be described below, and which carries theair under pressure supplied from a blower 367; each of these ducts385,386,387 permits injection into the associated buffer storage360,361,362 of air for fluidisation of the particles in it, the flowrate of this air being able to be adjusted individually by anappropriate valve 388 in the duct 385, 389 in the duct 386, 390 in theduct 387.

The particles are thus maintained, in each of the buffer storages360,361,362 in a state of fluidity such that they can be easily drawnoff by the drawing off means at a continuous adjustable flow rate onwhich this buffer storage 360,361,362 opens downwards; designated by369,370,371, are drawing off means associated respectively to the bufferstorage 360,361,362; each of these drawing off means 369,370,371 isadvantageously constituted by a rotary air lock or cellular distributor,having as is known a plurality of blades driven in rotation about anaxis, by a respective motor 372,373,374, inside an envelope with whichthese blades delimit cells which the rotation of the blades puts intocommunication alternatively with the associated buffer storage360,361,362, upwardly, and, downwardly, with a vertical duct 375,376,377for evacuation by gravity; the flow rate of such a cellular distributor,in terms of volume flow rate or mass flow rate, is controlled by thespeed of rotation of the blades, that is to say by their drive speed bythe associated motor 372,373,374.

Downwards, each of the ducts 375,376,377, opens into the duct 366mentioned above, approximately horizontal, in positions spaced along thelength of it downstream of the zone whence branch off the ducts385,386,387 for fluidisation air with reference to a direction 378 ofcirculation of air in this duct 366, imposed by the blower 367; adiaphragm 368 is interposed in the duct 366 between the opening of thedifferent ducts 375,376,377 and the opening of the ducts 385,386,387 forcausing a passage of air in these latter.

From this, the air carried by the duct 366, in accordance with a flowrate adjusted by adjustment of the blower 367, carries alongsuccessively the particles drawn off in the buffer storage 362 accordingto a flow rate determined by the cellular distributor 371, and whichfall via the duct 377, the particles drawn off in the buffer storage361, according to a flow rate determined by the cellular distributor370, and which fall via the duct 376, and the particles drawn off in thebuffer storage 360 according to a flow rate determined by the cellulardistributor 369, and which fall via the duct 375; it should be notedthat this order, chosen by way of example, is not a characteristic ofthe invention and as a result is not limitative on it; other manners ofconnection will in any case be described below, with reference to FIGS.3 and 4.

Downstream of the connection of all the ducts 375,376,377 with referenceto the direction 378, the air circulating in the duct 366 carries in thedirection 378 all the particles thus received as far as the injectionmeans 379 in all respects the same as the injection means 79 describedwith reference to FIG. 1, and arranged in the same manner as the latterwith respect to the grid 3, to the pipes 23 and 24, and to the level ofinjection 41 of larger particles separated by the first separation means36; in particular, the injection means 379 are oriented towards thetrajectory 20, and more precisely towards a part of it close to the gridin the upstream zone 21 of it, for favouring the carrying along of thefine particles thus injected at 379 by the coal projected by theprojector device 17 on the trajectory 20, and the following of thistrajectory as far as the grid 3 by these fine particles.

In conformity with the present invention, as well as the flow rate ofair in the duct 366, considered as a flow rate of conveying air takingaccount of the negligible characteristic of the part of this flow rateserving for fluidisation in the buffer storages 360,361,362, and theflow rate of particles in this air, via the drawing off means in thebuffer storages 360,361,362 here constituted by the cellulardistributors 369,370,371 are continuous, and the flow rate of particlesdownstream of the assembly of ducts 375,376,377, expressed in the termsof mass flow rate or volume flow rate, is at least approximatelyproportional to the load of the boiler, for example to the flow rate ofthe feed means 10 expressed in the same units.

For this, in conformity with the embodiment illustrated in FIG. 2, it isthe flow rate of each of the drawing off means in the buffer storages360,361,362, that is to say each of the cellular distributors369,370,371, which is thus controlled by the load of the boiler in amanner itself to be at least approximately proportional and, to thisend, a control of each of the motors 372,373,374 to the motor 16, hasbeen provided, in a manner to connect in a ratio of predeterminedproportionality the respective output speeds of these motors; thesecontrol means, shown by a connection in chain dotted lines 380, can bechosen by the man in the art from amongst a large range of possibilitiesand as a result will not be described.

By an appropriate adjustment of the ratio of proportionality, as afunction of the quantities of solid particles waiting in each of thedust removers 45,46,47 for the predetermined loads of the boiler takingaccount particularly of the characteristics of the coal used, a regularreinjection of these particles can thus be assured; it should be notedthat the ratio can be different for the different motors 372,373,374.

For permitting an absorption of the variations in the quantity ofparticles collected by the dust removers 45,46, 47 consecutively to therepercussion, with delay, of a variation in the load of the boiler oragain to a discharge of the dust removers, without perturbation of theconveyance by the duct 366 and without the reinjection into the fire boxat 379 causing excessive variations in the heat output, in additionthere is provided a control of the output speed of each of the motors372,373,374, that is to say the flow rate of the drawing off means369,370,371, to the variations of level in the respectively associatedbuffer storage 360,361,362, in comparison with the predetermined meanlevel 363,364,365; for this, there is provided means for correction ofthe control of the output speed of each of these motors, such that it isdefined by the means 380, as a function of the information provided bythe level detector 391,392,393 so that passing of the actual level ofparticles in one of the buffer storages above the mean predeterminedlevel causes a flow rate of the corresponding drawing off means369,370,371 greater than the flow rate calculated by proportionalitywith the load of the boiler, and on the contrary a reduction of thelevel below the predetermined level causes a reduction of the flow ratewith respect to the flow rate calculated by proportionality with theload of the boiler; it should be noted thus that, it is in additionassured that the drawing off means 369,370,371 receive particles, in thecorresponding buffer storage 360,361,362, an approximately constantforce permitting them to work in approximately constant conditions,independently of successive emptyings of the associated intermediaryhoppers.

The means permitting correction thus, step by step or continuouslyaccording to the type of level detector 391,392,393 used, the speed ofrotation of each of the motors 372,373,374 in a manner controlled to themeasurement of the level detector 391,392,393 associated to the samebuffer storage 360,361,362 have been simply shown by chain dottedconnections 381,382,383; as with the means 380, they can be chosen bythe man in the art from a large range of possibilities, withoutdeparting from the scope of the present invention.

The flow rate of particles in the duct 366 being thus determined, theflow rate of air in this duct, considered as a flow rate of conveyingair taking account of the small part of this flow rate which is drawnoff for fluidisation in the buffer storages 360,361,362 and preferablyconstant in terms of volume flow rate, is adjusted by action on theblower 367 so that the mass flow rate of the particles introduced intothe duct 366 is in a ratio to the mass flow rate of air in this duct, ofbetween 1 and approximately 10; these figures, given by way ofnon-limitative example, correspond to a high concentration of suspensionof particles in air injected at 379 in the boiler, such a highconcentration being favourable to the combustion of particles on theirarrival in the boiler and to their incineration to the form of ashesonce they are burned and find themselves on the grid 3.

Naturally, in addition to the characteristic arrangements of theinvention which have been described, the man of the art will provide allthe usual safety features and usual accessory arrangements; amongstthese accessory arrangements will be found particularly means (notshown) for emptying of the entire installation towards appropriate solidparticle storage means, and in particular means for emptying theseparators 45,46,47 but it will be noted that in place of being used inthe steady state as is the traditional case, these means will be usedexclusively in maintenance operations of the installation, the steadystate corresponding to a reinjection to the fire box 2 of all of theparticles extracted from the flue gases before their evacuation to theatmosphere by the means 44.

In addition, the man of the art can provide numerous variants of thedevice which has been described, without departing from the scope of thepresent invention; these variants can particularly be made in thepractical constitution of the second separation means 43, constituted inthe illustrated example by three fields of an electrostatic dust removerconnected in series by the flue gases duct 28; whatever their nature, adifferent number of these separators constituting the second separationmeans can be provided, and a different manner of mutual connection canbe provided, and FIGS. 3 and 4 illustrate precisely two modifications,in this direction, of the device illustrated in FIG. 2.

In the case of the variant illustrated in FIG. 3, a flue gas duct 128,corresponding to the ducts 328 and connected like it to anon-illustrated boiler, divides into parallel branches 128a and 128b ofwhich each is connected in series to two separators, respectively145a,146a as concerns the duct 128a, and 145b and 146b as concerns theduct 128b.

Each of these separators 145a, 146a,145b,146b has a respective lowerhopper 148a,149a,148b,149b opening downwards, via a respective valve151a, 152a, 151b, 152b, into a respective intermediary hopper154a,155a,154b,155b itself opening downwards, via a respective valve157a, 158a, 157b, 158b, into a respective buffer storage 160a, 161a,160b, 161b; this buffer storage opens itself downwards via continuousdrawing off means, with a controllable flow rate, such as a cellulardistributor respectively 169a,170a, 169b,170b, onto a higher end of avertical duct, respectively 175a,176a,175b,176b; these elements havingreference numerals resulting from a substraction of 200 with respect tothe reference numerals given to the elements just described withreference to FIG. 2, to which the elements in FIG. 3 are similar intheir structure, their inter-relation and their function.

In this variant, in spite of a branching of the separators145a,146a,145b,146b in series parallel, a single pneumatic conveyor duct166, in all respects comparable to the duct 366 described above and fedas it with air under pressure via a blower 167 in all respectscomparable to the blower 367, receives in a spaced manner the lower endsof the different ducts 176b,176a, 175a,175b, in this order, for carryingthe particles which it receives from these ducts, in suspension in theair, as far as the single injection means 179, in all respectscomparable to the means 379 described above, to the fire box of theboiler (not shown).

In the case of the variant illustrated in FIG. 4, there is found anassembly of elements illustrated in FIG. 3, having the referencesincreased by 100 with respect to the references that these elements havein FIG. 3, except that the single duct 166 and the single blower 167 arenot duplicated; more precisely, the ducts 275a and 276a, correspondingrespectively to the ducts 175a and 176a, open in a first air conveyorduct 266a and the ducts 275b and 276b corresponding respectively to theducts 175b and 176b open into a second pneumatic conveyor duct 266b,each of the ducts 266a and 266b having a first end connected to arespective blower 267a, 267b from it injecting conveying air with anadjustable and preferably constant flow rate, and a second end to whichthe two conveyor ducts 266a and 266b are connected in a single pneumaticconveyor duct 266 leading to the fire box of the boiler (not shown) viainjection means 279 in all respects comparable to the means 179, 79 or379, such as an injection pipe.

In the case of this variant, there could also be provided feeding of thetwo ducts 266a and 266b with conveying air in parallel, by means of asingle common blower 267 in place of the provision of a blower for eachof them and/or the providing of distinct paths between the two ducts asfar as the boiler, to the fire box of which they open then via injectionmeans 279a and 279b, in all respects comparable to the means 179, 79 or379, in place of there opening by means of the common injection means279; these two possibilities are shown in chain dotted lines in FIG. 4.

Naturally, in the case of these two variants, as in the case of theembodiment illustrated in FIG. 2, the number of separators crossed inseries by the flue gases, and the nature of these separators can bevaried in a large measure as a function of the needs estimated by theman in the art; in the case of these embodiments illustrated in FIGS. 3and 4 in addition, the number of branches from the flue gas duct 128 or228 can be greater than two, the ducts then corresponding to the ducts175a,176a,175b,176b or 275a,276a,275b,276b being able to open into asingle pneumatic conveyor duct of the type illustrated at 166 in FIG. 3,or into pneumatic conveyor ducts in parallel of the type illustrated at266a and 266b in FIG. 4, or again in series in the pneumatic conveyorducts branched in parallel.

Naturally, although the above description makes reference to a coalboiler, the scope of the invention will not be departed from by applyingit to boilers burning other solid fuels, as for example wood, husks,bagasses.

We claim:
 1. A device for reinjection of flown-off particles into asolid fuel boiler, fed with fuel by means (10) arranged in a first zone(9) of the boiler fire box and which projects continuously a determinedload of fuel on a trajectory (20) leading the latter into a second zone(21) of the boiler, onto a grid (3) driven with a return movement (8)from the second zone (21) towards the first (9), combustion occurringlong the said trajectory (20) and on the grid (3) permitting release(26) of flue gases entraining solid particles, this devicecomprising:means (27) for drawing off the flue gases in the boiler,means (44) for evacuation of the flue gases, first separation means(36), for the separation of relatively large particles, secondseparation means (43), for the separation of relatively fine particles,means (28) for leading the flue gases from the drawing off means (27) tothe first separation means (36), from the first separation means (36) tothe second separation means (43), from the second separation means (43)to the evacuation means (44) of the flue gases, means(37,38,39,40,41,42,84) for drawing off particles in the first separationmeans (36) and of reinjecting such particles into the boiler, means fordrawing off the particles in the second separation means (43),thisdevice being characterised in that the means for drawing off particlesin the second separation means (43) comprise: (a) a buffer storage(60,160,161,162,260,261,262, 360,361,362), (b) means (51 to 59) fordischarging particles from the second separation means (43) into thebuffer storage (60,160, 161,162,260,261,262,360,361,362), preventingdirect communication between these latter, (c) means(69,169,170,269,270,369,370,371) for continuous drawing off of particlesat the lower part of the buffer storage(60,160,161,162,260,261,262,360,361,362) with an adjustable flow rate,(d) means (80,81,380,381,382,383,391,392,393) for controlling to theload of the boiler the flow rate of the means(69,169,170,269,270,369,370,371) for continuous drawing off of particlesin the buffer storage (60,160, 161,162,260,261,262,360,361,362),and inthat there is provided: a source (98,107,42,167,267,367) of air underpressure, injection means (79,179,279,379) arranged in the proximity ofthe second zone (21) of the boiler and opening towards a part of thesaid trajectory (20) close to the grid (3) in this second zone (21), apneumatically conveying duct (66,166,266,366) for connecting the sourceof air under pressure (98,107,42,167,267,367) to the injection means(79,179,279,379), the means (69,169, 170,269,270,369,370,371) forcontinuous drawing off of particles in the buffer storage(60,160,161,162,260,261,262,360,361,362) opening into the said duct(66,166,266,366).
 2. A device according to claim 1, the secondseparation means (43) comprising a plurality of separators(45,46,47,145a,146a,145b,146b,245a,246a,245b,246b), connected in seriesand/or in parallel, between the first separation means (36) and themeans (44) for evacuation of flue gas, via the flue gas leading means(28,128,228), characterised in that the means of drawing off theparticles in the second separation means (43) comprise:(a) a pluralityof buffer storages (160a,161a,160b,161b,260a,261a,260b,261b,360,361,362) of which each is associated with atleast one of the said separators and placed below it, (b) means (51 to59,151a,152a,151b,152b,154a,155a, 154b,155b,157a,158a,157b,158b,251a,252a,251b,252b,254a,255a,254b,255b,257a,258a,257b,258) for discharging particles from this separator intothe associated buffer storage, preventing a direct communication betweenthese latter, (c) means(169a,170a,169b,170b,269a,270a,269b,270b,369,370,371) for continuousdrawing off of particles at the lower part of each buffer storage, withan adjustable flow rate, (d) means (380,381,382,383,391,392,393) forcontrolling to the load of the boiler the flow rate of each of the meansfor continuous drawing off of particles at the lower part of one of thebuffer storages,and in that the means for continuous drawing off ofparticles at the lower part of the different buffer storages open intothe said duct (66,166,266,366), which is common.
 3. A device accordingto claim 1, characterised in that the means for discharging particlesfrom the second separation means (43) into the buffer storage(60,160,161,162,260,261,262,360,361,362) comprise an air lock(54,55,56,154,155,156,254,255,256) with a small usable volume withregard to that of this buffer storage(60,160,161,162,260,261,262,360,361,362).
 4. A device according to claim2, characterised in that the means for discharging particles from oneseparator (45,46,47) into the associated buffer storage(60,160,161,162,260,261,262,360,361,362) comprise an air lock(54,55,56,154,155,156,254,255,256) with a small usable volume withregard to that of this buffer storage.
 5. A device according to claim 4,characterised in that the means (380,381,382,383,391,392,393) forcontrolling to the load of the boiler the flow rate of means(69,169,170,269,270,369,370,371) for continuous drawing off of particlesin the or each buffer storage (60,160,161,162,260,261,262,360,361,362)comprise means (381,382,383,391,392,393) for controlling this flow rateto maintain a mean level (363,364,365) of particles in this latter.
 6. Adevice according to claim 1, characterised in that it comprises means(385 to 390) for fluidisation of particles in the or each buffer storage(360,361,362).
 7. A device according to claim 1, characterised in thatthe means (69,169,170,269,270,369,370,371) for drawing off the particlesin the or each buffer storage (60,160,161,162,260,261,262,360,361,362)comprises a cellular distributor.
 8. A device according to claim 1,characterised in that it comprises:a second source (98) of air underpressure, a second pneumatic conveying duct (97) connecting this secondsource (98) to the buffer storage (60), the said means (51 to 59) fordischarging particles from the second separation means (43) into thebuffer storage (60) opening into this second duct (97) whilst preventingdirect communication between this latter and the second separation means(43).
 9. A device according to claim 8, the second separation means (43)having a plurality of separators (45,46,47), connected in series and/orin parallel, between the first separation means (36) and the means (44)for evacuation of flue gas, via the flue gas leading means (28),characterised in that the buffer storage (60) is a single one, in thatit is provided with means (51 to 59) for discharging particles from eachof the separators (45 to 47) into the single buffer storage (60), thesedischarging means opening into the said second duct (97), which iscommon, whilst preventing direct communication between this latter andthe separators.
 10. A device according to claim 8, characterised in thatthe means for drawing off particles from the second separation means(43) into the buffer storage (60) comprises an air lock (54,55,56) witha small usable volume with regard to that of this buffer storage (60).11. A device according to claim 9, characterised in that the means fordrawing off particles from one of these separators (45,46,47) into thesingle buffer storage (60) comprises a respective air lock (54,55,56),the combined usable volume of these air locks being less than that ofthis buffer storage.
 12. A device according to claim 10, characterisedin that it comprises means (100 to 105) for fluidisation of particles inthe or each air lock (54,55,56).
 13. A device according to claim 8,characterised in that the means (80) for controlling to the load of theboiler the mean flow of means (69) for continuous drawing off ofparticles in the buffer storage (60) comprises means (81,91) forcontrolling this flow to maintain a mean level (63) of particles in thislatter.
 14. A device according to claim 8, characterised in that itcomprises means (85,88) for fluidisation of particles in the bufferstorage (60).
 15. A device according to claim 8, characterised in thatthe means (69) for drawing off particles in the buffer storage (60)comprises a cellular distributor.
 16. A device according to claim 8,characterised in that the first source (98,107) is constituted by anupper part (107) of the buffer storage (60) and by the second source(98).
 17. A device according to claim 8, characterised in that there isprovided means for return (66b) of gas from the buffer storage (60) tothe second separation means (43), and in that the two sources (98; 42)are disassociated.
 18. A device according to claim 8, characterised inthat the buffer storage is open to free air via a filter (66b), and inthat the two sources (98, 42) are disassociated.